-Putting a fish into excess salt water upsets the balance in osmosis, in which cells naturally try to balance the level of salt/other stuff and water inside and outside the membrane. If the fish lives in fresh water, and is dropped into salt water, its outer cells (eyes, mouth, around the scales) will try to get the inside of the cell to match the concentration of salt in the water. This will cause the cells to expel water (salt does not pass though the membranes) and they will shrivel up. Needless to say, this is often fatal for the fish, as this process occurs wherever it is hit by the water. It will live for a while before this occurs, so for very short periods (minutes) it can painfully live in the alternate environment.

If you put a saltwater fish into fresh water, the alternate case occurs, and the cells absorb water until they burst, so creating the same outcome, death of fish.